[0001] This invention relates to liquid crystal devices and is particularly concerned with
liquid crystal devices in which the liquid crystal material can be selectively controlled
by the application of an electric or magnetic field to vary the light transmissivity
of the device, as in an optical shutter for example.
[0002] It is known from, for example, French Patent No 2139537, US Patent No 4435047 and
US Patent 4671618 to encapsulate a nematic liquid crystal material as a multiplicity
of droplets within a matrix of transparent, encapsulating material across which an
electric or magnetic field can be selectively applied. In the field-off state, the
nematic liquid crystal molecules in each capsule tend to align with the three dimensional
boundary wall of the encapsulating material bounding the capsule so that collectively,
the liquid crystal molecules do not have any preferred overall direction of alignment.
In this condition, the liquid crystal material is insensitive to the polarisation
of light incident on the device and the device is then substantially opaque.
[0003] When a field of suitable magnitude is applied to the device, the tendency of the
liquid crystal molecules to align with the boundary walls of the capsules is overcome
and the molecules under the influence of the applied field tend to re-orientate into
generally parallel alignment and in this condition light can be transmitted through
the device.
[0004] It is also known from US Patent No 4411495 that selective light transmission through
a liquid crystal device can be effected by dispersing a liquid crystal material within
a layer comprising a porous filter material made of mixed esters of cellulose and
controlling the refractive index of the liquid crystal material by the application
of an electric field to produce conditions in which the refractive indices of the
liquid crystal and filter material are matched or mismatched. When the refractive
indices are matched, the device is transmissive and, when they are mismatched, the
device becomes opaque to incident light. It will be noted that refractive index changes
are fundamental to the operability of devices constructed in accordance with US Patent
No 4411495.
[0005] According to a first aspect of the present invention there is provided a liquid crystal
device comprising containment means enclosing liquid crystal material, means for applying
an electric or magnetic field across the liquid crystal material and a permeable body
of optically non-absorbing material permeated by the liquid crystal material such
that light transmission through the composite comprising said body and the liquid
crystal material is reduced in the off state of said field-applying means, characterised
in that said permeable body comprises fibres or filaments deposited to form a layer.
[0006] According to a second aspect of the present invention there is provided a liquid
crystal device comprising containment means enclosing liquid crystal material, means
for applying an electric or magnetic field across the liquid crystal material, and
a permeable body of optically absorbing material permeated by the liquid crystal such
that light transmission through the composite comprising said body and the liquid
crystal material is reduced in the off state of said field-applying means, characterised
in that said permeable body comprises fibres or filaments having diameters such that,
in the field-on liquid crystal-aligned state of the device, the light transmissivity
of said composite is substantially insensitive to differences of up to 0.01 in the
ordinary refractive indices of the liquid crystal material and said fibres/filaments.
[0007] As noted above, in the device of US Patent No 4411495, light transmissivity is governed
by matching/mismatching of the refractive indices. The same also applies to the encapsulating
approach taught in French Patent No 2139537 and US Patents Nos 4435047 and 4671618.
In a device according to the present invention, light transmissivity in the field-on
state can be made less sensitive to refractive index mismatch by appropriate selection
of the fibre/filament diameters, ie the present invention is based on the recognition
that the light-scattering required in the field-off state of the device may be provided
by means of a permeable body comprising small diameter fibres or filaments while securing
a high degree of optical clarity in the field-on state even when a refractive index
mismatch exists. This is because the light scattering effect caused by refractive
index mismatch is related to fibre or filament diameter and can be reduced if the
fibre/filament diameters are on the sub-micronic scale.
[0008] Of course, while light scattering is an undesirable effect in the field-on state,
it is necessary in the field-off state to provide a light shutter or valve effect.
In a device according to the invention, in the field-off state less reliance is placed
on light scattering due to refractive index mismatch - indeed the small fibre/filament
diameters inherently lead to reduced light scattering by this mechanism. Instead,
advantage is taken of another light-scattering mechanism which comes into play in
the field-off state when the fibre/filament diameters are appropriately small. This
latter mechanism is believed to arise from the formation within the liquid crystal
material of domains in which the local refractive index is different from adjacent
domains because the liquid crystal molecules are differently aligned, such domain-formation
being promoted, it is believed, by the production of disclinations when small diameter
fibres or filaments are introduced into the liquid crystal material.
[0009] Thus, according to another aspect of the present invention, a liquid crystal device
is characterised in that the permeable body comprises fibres or filaments which are
so dimensioned and distributed that, in a reduced transmissivity state of the composite
comprising the liquid crystal material and said fibrous or filamentary permeable body,
the fibres/filaments are instrumental in the formation of domains in the liquid crystal
material on a scale such that at least 25% of the scattering of incident light by
the composite is attributable to the formation of said domains. Preferably at least
40% of the light scattering is attributable to domain formation and more preferably
the light scattering in the field-off state is attributable predominantly to the formation
of domains as a consequence of the presence of appropriately small diameter fibres/filaments.
[0010] The liquid crystal material is conveniently nematic but may be cholesteric, nematic
doped with cholesteric, or smectic.
[0011] The fibre/filament diameters are preferably such that, in the field-on, fully aligned
state, the light transmissivity of the composite varies by no more than 10% (more
preferably 5%) in response to ordinary refractive index differences ranging up to
0.01 (up to 0.02 and beyond is also feasible within the scope of the invention). As
used herein, the term "light transmissivity" is used to refer to the quantity 100%
- haze (%) where the haze value is measured in the manner described hereinafter.
[0012] Because the constraint of precise refractive index matching can be relaxed, in a
typical device according to the invention, there may be a refractive index mismatch
present (for example 0.012 or even larger) coupled with a light transmissivity of
at least 90% in the field-on, fully aligned state.
[0013] By "fully-aligned" is meant a state in which the magnitude of the applied field is
sufficient to align substantially all of the liquid crystal molecules with the applied
field direction.
[0014] In the field-off state, the transmissivity of the composite is preferably less than
30% (more preferably 20%).
[0015] Preferably at least a major proportion of said fibres/filaments have sub-micronic
diameters. Advantageously, at least 50% (more preferably 70%) of the fibres/filaments
have diameters not greater than 500 nanometer (nm), more preferably 300 nm.
[0016] The permeable body is conveniently in the form of a mat of fibres or filaments, which
may be produced by a spinning technique, eg electrostatic, centrifugal or blow spinning.
[0017] In a specific embodiment of the invention, the containment means includes a pair
of spaced boundary walls, at least one of which is transparent, both boundary walls
being provided with electrode means whereby an electric field can be applied across
the space between them. It is not necessary in all cases for both boundary walls to
be transparent, one may for example be reflective.
[0018] The permeable fibrous or filamentary body is enclosed within the space between the
boundary walls and is impregnated with the liquid crystal material. The overall thickness
of the liquid crystal layer and the permeable body is typically 1-40 microns, eg 10
microns.
[0019] The fibrous or filamentary body may initially be enclosed between the boundary walls
and the liquid crystal material may be subsequently introduced into the fibrous or
filamentary body, for example, by introducing the liquid crystal material at one or
more points along the edge or edges of the assembly comprising the fibrous or filamentary
body/boundary walls and, in this case, the permeable body may act as a wick to aid
distribution of the liquid crystal throughout substantially the entire extent of the
permeable body.
[0020] In another embodiment, the permeable body may be deposited on one of the layers of
material which is to constitute one of the boundary walls; the liquid crystal may
then be applied to the permeable body to fill the voids thereof and the second boundary
wall may thereafter be assembled to enclose the liquid crystal-impregnated permeable
body. Application of the liquid crystal to the permeable body may be effected by,
for example, a coating apparatus in which the liquid crystal is spread over the permeable
body, while the latter is exposed, by means of a rotating roller which dips into a
reservoir of the liquid crystal. Alternatively, the liquid crystal may be applied
by spraying it on to the exposed permeable body.
[0021] In a presently preferred method, the permeable body is formed in situ on one of the
boundary walls, prior to assembly of the boundary walls with the permeable body, by
spinning fibres on to said one boundary wall to form a mat. Spinning may be effected
by any suitable technique, for example, electrostatic, centrifugal or blow spinning.
[0022] The fibres can be melt spun, but preferably are spun from solutions of fibre-forming
polymers chemically inert with respect to the liquid crystal material. The structure
of the fibrous mat thus produced can be stabilised in the former case by arranging
conditions such that the fibres are still soft when they fall on each other and, in
the latter case, such that they still contain some solvent. Fibre-forming materials
which cure in flight, either through contact with air or other ambient gas, or through
appied heat or radiation, eg ultraviolet light, or through mixing of reactive precursors
during spinning, can also be used, and again stabilisation of the resulting mat can
be achieved by contacting the fibres before cure is complete. Fusion or bonding between
the fibres themselves is generally preferred to application of a size or other bonding
agent to the loosely-formed mat, although this latter does provide an alternative
useful in some applications. Generally suitable polymers for solution spinning are
polyvinyl alcohol and polyvinyl butyral, both of which can be sprayed from a solution
in a water/methanol mixture, to produce a mat substantially chemically inert to most
common liquid crystals. Other examples include polyvinyl chloride, polyvinyl formal,
various cellulose derivatives, polystyrene, polymethyl methacrylate. polyether imide
and polyether sulphone.
[0023] Examples of polymers that can be used in melt spinning fibres include polypropylene,
polyethylene and terephthalate.
[0024] In another embodiment of the invention, the fibres can be formed from polymers which
are produced by the polymerisation or cross-linking of monomers or oligomers prior
to or following spinning of the fibre. Examples of monomers and oligomers which can
be formed into polymers either prior to or following spinning include epoxy resin,
UV curable and/or thermocurable acrylic monomers and oligomers, and vinyl monomers
and oligomers.
[0025] The material of which the boundary walls are composed may have a refractive index
which matches either the liquid crystal material or the material of the fibres or
filaments. This may be achieved for example by fabricating the walls and the fibres
from the same material or materials which have subtantially the same refractive index.
[0026] The invention will now be described with the aid of the embodiment illustrated by
way of example only in the accompanying drawings, and also with reference to the Examples
that follow.
[0027] In the drawings:
Figure 1 is a schematic view of an optical shutter-type liquid crystal device;
Figures 2 and 3 are graphs illustrating qualitatively variation of haze with refractive
index difference.
[0028] Referring to Figure 1, the device illustrated is an optical shutter which is switchable
between a state in which it is substantially transparent to incident light and a state
in which it is substantially opaque. The device comprises a containment cell defined
by upper and lower boundary walls 11, 12 and sides (not shown). The walls 11, 12 are
composed of a flexible transparent plastics material, the contiguous faces of which
each have a layer 13, 14 of a transparent conductive material such as indium tin oxide
applied thereto whereby an electrical field can be applied between the electrodes
13, 14. The walls 11, 12 are spaced apart and enclose therebetween a skeletal fibrous
structure 15, the intersecting fibres of which define a myriad of interconneced voids
filled with liquid crystal material so that the space between the boundary walls is
occupied by a layer of liquid crystal material in which the fibrous structure 15 is,
in effect, immersed. The fibrous structure 15 may be produced as a mat by a suitable
spinning technique.
[0029] The fibres are formed with sub-micronic diameters so that the need for close matching
of the ordinary refractive indices of the liquid crystal material and the material
from which the fibres are formed can be relaxed while allowing good contrast to be
achieved between the transparent and opaque states of the device. Figures 2 and 3
illustrate qualitatively the variation of haze (which is inversely related to transmissivity)
with the difference in ordinary refractive indices of the liquid crystal and the fibres
when the electric field is applied, via electrodes 13, 14, to orient the liquid crystal
molecules with the field direction. In the case of Figure 2, the fibres are of a relatively
large diameter such that the refractive index mismatch has a significant influence
on transmissivity. In the case of Figure 3, in accordance with the invention the fibres
have sub-micronic diameters. In both cases, it will be seen that the haze is at a
minimum (corresponding to maximum transmissivity) when the refractive indices of the
liquid crystal and the fibres are precisely matched. However, as the refractive index
difference increases, the increase in haze is much more marked in the case of Figure
2 and consequently the transmissivity of a device having larger fibre diameters is
more sensitive to refractive index mismatch.
[0030] Before proceeding to the Examples, the following terms which are used herein will
be explained.
"Haze values" are given as percentages - Haze (0V) and Haze (100V) for instance correspond
to values measured with voltages of zero volts (i.e. no applied field) and 100 volts
applied between the electrodes of the device, haze values being measured in accordance
with ASTM Standard D1003 by means of a Hazemeter as manufactured by Tokyo Instrument
Company Limited of Japan.
"Contrast" - this is calculated as [Haze (OV) - Haze (100V)]/Haze (OV).
"Average Fibre Diameter" - a statistical average of diameter values obtained from
sample measurements using scanning electron microscopy (SEM).
"Percentage of fibres having a diameter less than X nm" - a statistical computation
based on sample measurements using SEM.
"n
o": ordinary refractive index of liquid crystal materials.
"n
p-n
o": refractive index difference between the fibre material and the liquid crystal.

Example 1
[0031] Using polyvinyl butyral (PVB) (obtained from Hext Co. Ltd, BGOT) as the polymer for
fibre formation, this was dissolved in isopropyl alocohol to obtain a 6% solution.
0.25g of Coronate HL (obtained from Nippon Polyurethane Co. Ltd of Japan) to be abbreviated
as NPU) was added as the cross-linking agent to 50g of the polyvinyl butyral solution
followed by shaking until uniformly dissolved. An indium oxide (15:5) based transparent,
conductive film was then sputtered to a thickness of 500 Å on to a polyester film
which was then cut into 7cm x 7cm pieces having thickness of 100 µm. The above PVB
solution was then dispersed for three minutes onto the above conductive polyester
film using an electrostatic spinning apparatus, the flow rate of the above polymer
solution being 2.0 cc/hr and the nozzle voltage being 25 KV DC, to obtain a transparent
fibre mat/polyester film assembly. This assembly was then placed in an oven and allowed
to stand for 1 week at 50°C to perform cross-linking treatment of the PVB. Upon measuring
the fibre diameters of the fibre mat/film assembly that was obtained following cross-linking
treatment, using scanning electron microscopy, the average diameter was found to be
0.32 µm and the percentage of fibres having a diameter of 0.5 µm or less was 63%.
Following this, various liquid crystals (products of the Merck Co Ltd) having different
refractive indices indicated in Table 1 were permeated into the fibre mat. Following
permeation of the liquid crystals, another polyester film with a transparent, conductive
film was placed on the existing film to sandwich the liquid crystal filled fibre mat.
The electro-optical characteristics of the liquid crystal display element obtained
in this manner are indicated in Table 1. As is clear from Table 1, Haze (100) did
not exhibit major fluctuations with respect to changes in the difference in refractive
indices between the liquid crystal and fibre, and indicated values that were somewhat
low (3.5-5.1%). Therefore, in the case of liquid crystal display elements that were
produced using small diameter fibres, it was found that restrictions on the difference
in refractive indices of the fibre material and the liquid crystal could be relaxed
considerably. In addition, it can also be seen from Table 1 that good contrast values
were obtained.
Example 2
[0032] Other than increasing the polymer concentration from 6% to 10%, a cross-link treated
fibre mat/film assembly was produced using the same prescription as Example 1. Upon
measurement of fibre diameter. the average diameter was 0.87 µm and the percentage
of fibres having a diameter of 0.5 µm or greater was 18%. After respectively permeating
the resulting fibre mats with Merck liquid crystals ZLI 1289 (n
o: 1.517), E-37 (n
o: 1.522) and E-49 (n
o: 1.527), the liquid crystal display device was produced in the same manner as that
of Example 1. The electro-optical characteristics of the liquid crystal display obtained
in this manner are indicated in Table 1. As is clear from Table 1, in all of these
cases. Haze (100) demonstrated low values. In addition, good contrast values were
obtained.
Example 3
[0033] Using polyvinyl alcohol (PVA) BDH Co. Ltd. MW: 125,000) as the polymer, this was
dissolved in a solution consisting of isopropyl alcohol and water (mixture ratio:
50%:50%) to obtain a 3.5% solution. After preparing the conductive polyester film
in the same manner as Example 1, the above solution was then dispersed for three minutes
onto the above conductive polyester film using an electrostatic spinning apparatus,
at a flow rate of the above polymer solution of 2.0 cc/hr and with a nozzle voltage
of 28KV DC, to deposit on the polyester film a transparent, fibre mat. After curing
the fibres in the same manner as Example 1, upon measuring the fibre diameters of
the above film using scanning electron microscopy, the average diameter was found
to be 0.35 µm, and the percentage of fibres having a diameter of 0.5 µm or less was
90%. After various liquid crystals (products of the Merck Co Ltd) having different
refractive indices indicated in Table 2 were permeated into the fibre mat, the liquid
crystal display element was produced in the same manner as Example 1. The electro-optical
characteristics of the liquid crystal display element obtained in this manner are
indicated in Table 2. As is clear from Table 2, Haze (100) demonstrated low values
(2.2-5.5%) without exhibiting major fluctuations with respect to changes in the difference
in refractive indices between the liquid crystal and fibre. Therefore. in the case
of liquid crystal display elements that were produced using small diameter fibres,
it was found that restrictions on the refractive index of the liquid crystal could
be relaxed considerably. In addition, good contrast values were obtained.
TABLE 1
EXAMPLE NO. |
FIBRE |
LIQUID CRYSTAL |
[np-no] (absolute value) |
HAZE (100V)% |
HAZE (0V)% |
HAZE DECREASING RATIO (%) |
CONTRAST |
|
AV. DIA (µm) |
% LESS THAN 0.5 µm |
GRADE |
no |
|
|
|
|
|
1-1 |
0.32 |
63 |
ZLI 2214 |
1.490 |
0.025 |
4.8 |
80.8 |
81.5 |
0.94 |
1-2 |
" |
" |
ZLI 1691 |
1.499 |
0.016 |
4.2 |
76.3 |
80.5 |
0.94 |
1-3 |
" |
" |
ZLI 3238 |
1.505 |
0.010 |
4.3 |
83.2 |
85.2 |
0.95 |
1-4 |
" |
" |
ZLI 1289 |
1.517 |
0.003 |
3.5 |
90.2 |
87.2 |
0.96 |
1-5 |
" |
" |
E-37 |
1.522 |
0.007 |
4.9 |
78.3 |
86.3 |
0.94 |
1-6 |
" |
" |
E-49 |
1.527 |
0.012 |
5.1 |
92 |
70.3 |
0.95 |
2-1 |
0.87 |
18 |
ZLI 1289 |
1.517 |
0.003 |
4.9 |
90.8 |
90.2 |
0.95 |
2-2 |
" |
" |
E-37 |
1.522 |
0.007 |
5.1 |
76.9 |
83.6 |
0.93 |
2-3 |
" |
" |
E-49 |
1.527 |
0.012 |
10.0 |
83.3 |
67.7 |
0.87 |
TABLE 2
EXAMPLE NO. |
FIBRE |
LIQUID CRYSTAL |
[np-no] (absolute value) |
HAZE (100V)% |
HAZE (0V)% |
HAZE DECREASING RATIO (%) |
CONTRAST |
|
AV. DIA (µm) |
% LESS THAN 0.5 µm |
GRADE |
no |
|
|
|
|
|
3-1 |
0.35 |
90 |
ZLI 2214 |
1.490 |
0.029 |
4.0 |
63 |
83.5 |
0.94 |
3-2 |
" |
" |
ZLI 1691 |
1.499 |
0.020 |
3.5 |
80.8 |
81.2 |
0.96 |
3-3 |
" |
" |
ZLI 3238 |
1.505 |
0.014 |
3.3 |
72.2 |
84.4 |
0.95 |
3-4 |
" |
" |
ZLI 1289 |
1.517 |
0.002 |
2.2 |
80.9 |
91.4 |
0.97 |
3-5 |
" |
" |
E-37 |
1.522 |
0.003 |
5.3 |
83 |
91.4 |
0.94 |
3-6 |
" |
" |
E-49 |
1.527 |
0.008 |
5.5 |
90 |
65.1 |
0.94 |
1. A liquid crystal device comprising containment means enclosing liquid crystal material,
means for applying an electric or magnetic field across the liquid crystal material
and a permeable body of optically non-absorbing material permeated by the liquid crystal
material such that light transmission through the composite comprising said body and
the liquid crystal material is reduced in the off state of said field-applying means,
characterised in that said permeable body comprises fibres or filaments deposited
to form a layer.
2. A device as claimed in Claim 1 in which said fibres or filaments have diameters
such that, in the field-off state, domain formation is promoted in the liquid crystal
material on a scale such that at least 25% of the light attenuation in the off-state
is attributable to light scattering as a result of domain formation.
3. A liquid crystal device comprising containment means enclosing liquid crystal material,
means for applying an electric or magnetic field across the liquid crystal material,
and a permeable body of optically absorbing material permeated by the liquid crystal
such that light transmission through the composite comprising said body and the liquid
crystal material is reduced in the off state of said field-applying means,
characterised in that said permeable body comprises fibres or filaments having diameters
such that, in the field-on liquid crystal-aligned state of the device, the light transmissivity
of said composite is substantially insensitive to differences of up to 0.01 in the
ordinary refractive indices of the liquid crystal material and said fibres/filaments.
4. A device as claimed in Claim 3 in which said fibre/filament diameters are such
that, in the field-on, aligned state the light transmissivity of said composite varies
by no more than 10% in response to ordinary refractive index differences ranging up
to 0.01.
5. A device as claimed in Claim 3 in which said fibre/filament diameters are such
that, in the field-on, aligned state the light transmissivity of said composite varies
by no more than 5% in response to ordinary refractive index differences ranging up
to 0.01.
6. A device as claimed in any one of Claims 3 to 5 in which said insensitivity is
exhibited for ordinary refractive index differences ranging up to 0.02.
7. A device as claimed in any one of Claims 1-6 in which the ordinary refractive indices
of the liquid crystal material and said fibres/filaments differ and the light transmissivity
of said composite is at least 90% in the field-on liquid crystal-aligned state.
8. A device as claimed in Claim 7 in which said refractive indices differ by at least
0.005.
9. A device as claimed in Claim 7 in which said refractive indices differ by up to
0.01.
10. A device as claimed in claim 7 in which said refractive indices differ by up to
0.012.
11. A device as claimed in any one of Claims 1-10 in which at least a major proportion
of said fibres/filaments have sub-micronic diameters.
12. A device as claimed in any any one of claims 1-10 in which 50% of said fibres/filaments
have diameters no greater than 500nm.
13. A device as claimed in any one of Claims 1-10 in which at least 70% of said fibres/filaments
have diameters no greater than 500nm.
14. A device as claimed in any one of Claims 1-13 in which the fibres/filaments have
an average diameter less than 1 micron.
15. A device as claimed in any one of Claims 1-13 in which the fibres/filaments have
an average diameter less than 750 nm.
16. A device as claimed in any one of Claims 1-13 in which the fibres/filaments have
an average diameter less than 500 nm.
17. A device as claimed in any one of Claims 1 to 16 in which said permeable body
comprises a mat of said fibres or filaments.
18. A device as claimed in in Claim 17 in which said mat is produced by spinning.
19. A device as claimed in any one of Claims 1-18 in which, in the field-off state,
the transmissivity of said composite is less than 30%.
20. A device as claimed in any one of Claims 1-19 in which, in the field-off state,
the transmissivity of said composite is reduced predominantly by light scattering
by domains formed in the liquid crystal material as a consequence of the presence
of sub-micronic fibres or filaments therein.